When we burn fuel, we need to mix it with air so it can burn properly. Just like you need the right amount of milk and chocolate when making hot chocolate, we need the right amount of air and fuel for the engine to work right.
The air to fuel ratio is the balance between the amount of air and fuel that we mix together. If we have too much air and not enough fuel, the engine won't work properly because it won't have enough fuel to burn. If we have too much fuel and not enough air, the engine won't work either, because it will have too much fuel and not enough oxygen to burn it.
Just like a chef needs to measure ingredients when cooking to make sure things taste good, car mechanics and engineers need to measure the right amount of air and fuel to make sure that the car runs smoothly and uses fuel efficiently.